The Trump Institute was founded in the mid-2000s and organised seminars in hotel ballrooms, where attendees would pay up to $2,000 to hear Mr Trump's "wealth-creating secrets and strategies".

The New York-born billionaire recorded an advert, broadcast in 2005, which promised that the Institute would give access to his vast accumulated knowledge.

"I put all of my concepts that have worked so well for me, new and old, into our seminar," he said. "I'm teaching what I've learned."

But a Democrat super-PAC, American Bridge, has investigated the Institute and found that the "concepts" touted by Mr Trump actually came from a decade-old guide published in 'Success' magazine, called Real Estate Mastery System.

At least 20 pages of the Trump Institute book were copied entirely or in large part from the 1995 guide.

The story of the Institute bears strong parallels with Trump University, which has seen angry students take legal action to get their money back. The cases will not be heard until after November's election.

Trump University was 93pc owned by Mr Trump himself, but the Trump Institute was owned by Irene and Mike Milin, a couple who had been marketing get rich quick courses since the 1980s.

Mr and Mrs Milin had been accused of fraud by Texas, Florida and Vermont since the early 1990s, and, in 2007, 33 state attorneys general signed a letter to the Federal Trade Commission accusing the Milins of deceptive trade practices. A year later, their company sought bankruptcy protection, owing $2.1m to creditors.

Ms Milin, reached by phone at her home in Florida, told 'The New York Times' she was too busy to comment, and hung up.